A recent article (S. S. Bleehen: J. Soc. Cosmet. Chem 28 407-412, 1977) aptly stated "Hyperpigmentation of the skin in man can be the cause of much mental distress and hypermelanotic areas, particularly on the face, can result in marked cosmetic disability." While the statement is made in the context of men, it is even more appropriate with regard to women, who, in addition to non-sex-related melanotic blemishes, are also subject to hypermelanotic chloasmas as a result of child-bearing (chloasma gravidarum) and birth-control pills (chloasma pillularae). The prime cosmetic solution to those conditions has been the application of "bleaching" lotions and creams to affected areas.
These lotions and creams are emulsions (W/O or O/W) wherein 1 to 5 wt.% of hydroquinone is dissolved in the aqueous phase. Due to the liquid or semi-liquid nature of the vehicle the hydroquinone is generally distributed over the widely affected areas of the body.
To be effective, the hydroquinone must be absorbed into the lower layers of the skin, in order to inhibit, at the melanocytes, the formation of the enzyme tyrosinase. This enzyme converts the amino-acid tyrosine into melanin, the coloring pigment of the blemish. The use of hydroquinone via aqueous emulsified vehicles is well-known, as seen in U.S. Pat. No. 3,856,934 which describes the combination of hydroquinone with corticosteroids and the keratolytic agent, Vitamin A acid.
This patent and the above mentioned article include an excellent review of the literature of cosmetic lightening (bleaching) of the skin. It is generally recognized that hydroquinone is the least irritating of the tyrosine-inhibiting agents including ammoniated mercury, 4-isopropyl catechol(4-IPC), and hydroquinone monobenzyl ether. Problems with these other agents are their systemic effects. Mercury salts, of course have recognized toxic effects. However the hydroquinone monobenzyl ether and 4-IPC have broad systemic depigmenting effects due to their melanocytotoxic effects. Hydroquinone is the least irritating and has the least amount of systemic effects.
All prior cosmetic preparations containing hydroquinone have been based upon its cream or lotion application due to the solubility limitations imposed by hydroquinone which required the use of water as the solvent. Hydroquinone is soluble in water in a 1:14 ratio. This solution is then the aqueous phase component of the creams and lotions. However aqueous solutions of hydroquinone have finite stability which limits the shelf-life of these previous cosmetic preparations.
In the treatment of skin spots, particularly "Age Spots" "Pill Spots" and "Liver Spots", it would be most desireable to localize the application of the hydroquinone. Application from a solid stick would be most desireable for circumscribed application to the spots or blemishes.
Alcoholic-gel cosmetic sticks of the type popularly used in stick deodorants would appear to be a good base for such cosmetic bleaching sticks since hydroquinone is quite soluble in alcohol. However, the gelling agents in such sticks is sodium stearate, a soap, but the alkalinity of soaps readily and rapidly decomposes hydroquinone. For this reason hydroquinone "bleaching" sticks have not heretofore appeared on the market.